In the course of their production, photographic materials are typically chilled and stored in the gelled state following preparation in order to prevent qualitative degradation. It is then necessary to liquefy the gelled materials so they can be coated on a film or paper support. Gelled photographic materials include aqueous or solvent based photosensitive or non-photosensitive emulsions or dispersions.
Two general methods for liquefying gelled photographic materials are known.
In the batchwise method, gelled photographic material is loaded into a tank which is fitted with a stirring means. Heat is provided to the exterior of the tank, while the material is stirred inside. All of the material in the tank is melted at one time, and then drawn off as needed.
The batchwise method has serious drawbacks, because an entire batch of gelled material is melted at a time, causing individual increments of gel to be overheated. The result is qualitative degradation of the material and varying sensitometry along the length of the coated film.
Alternatively, the gelled material may be continuously liquefied by any of several known methods. In one such continuous liquefaction method, the gelled material is loaded into a hopper, pumped from the hopper into a vacuum drum where entrapped air is removed, and then pumped into a heat exchanger. The material is melted in the heat exchanger and conveyed to a surge pot, from which it is delivered to a coating apparatus.
Several disadvantages are associated with the use of this method, however. The vacuum drum is needed to remove air which enters the system through the upstream pump system. Unfortunately, the presence of the vacuum drum causes material discharged from the downstream pumping system to flow back toward the vacuum drum. As a result, large pressure surges occur downstream of the vacuum drum. These conditions necessitate the use of the surge pot to dampen pulsations prior to delivery to the coating apparatus. However, the vacuum drum and surge pot increases the size and hold-up volume of the apparatus, resulting in excessive waste of material and difficult and time-consuming cleaning procedures. In addition, this method is useless for liquefying small amounts of material, because the entire length of the system must be filled with material in order to operate. Achieving and maintaining sufficient vacuum in the vacuum drum is another concern associated with this method. Also, the pumps used in this system tend to impart unacceptably high shear levels to the gelled material which causes unacceptable sensitometry degradation.